Cold-curing silicone rubber materials, also referred to as “RTV” (from German “Raumtemperatur-vernetzende”=cross-linking at room temperature) silicone rubber materials, have been known for quite some time as custom-designed materials which have elastic properties. They are used, generally, as sealants or adhesives for glass, porcelain, ceramics, stone, plastics, metals, wood, etc., e.g. in applications like joint filling and sealing compounds in construction and sanitary installations, or as coating agents, e.g. in the electronics industry (Röimpp Chemie Lexikon, CD ROM, version 2.0, ed. J. Falbe, Thieme-Verlag, Stuttgart 1999; as well as Ullmanns Enzyklopädie der Technischen Chemie, 4th edition, ed. E. Bartholome, Verlag Chemie, Weinheim 1982, vol. 21, p. 511 et seq.). Use is made especially of single-component RTV silicone rubber materials (RTV-1); These are, for example, plastically mouldable mixtures made of α,ω-dihydroxy-polyorganosiloxanes and appropriate cross-linkers (also referred to as cross-linking agents or hardeners in the art), which are suitable for storing under exclusion of moisture (e.g. within a suitable cartridge) but polymerize under the influence of water or humidity of the air at room temperature. Polymerization, as a rule, takes place by condensation of SiOH groups with appropriate hydrolyzable SiX groups of the cross-linkers.
Depending on the desired chemical and physical properties of the polymerization product, such as, e.g., the desired degree of cross-linking, the solvent resistance, etc., various polyfunctional cross-linkers (hardeners), e.g. tri-funtional and/or tetra-functional cross-linkers (hardeners), are usually used together with various polyorganosiloxanes which either are difunctional or carry more functional groups. The most frequently chosen difunctional polyorganosiloxane compounds are α,ω-dihydroxy-polyorganosiloxanes.
Based on the leaving groups (HX) released by the hydrolyzis of the cross-linker, a distinction is made with RTV-1 silicone rubber materials between acid systems (HX=acids, such as, e.g., acetic acid, etc.), basic systems (e.g. HX=amines, etc.), and neutral systems (e.g. HX=alcohols, oximes, etc.). RTV-1 silicone rubber materials which are commercially available at this time usually comprise acid systems which hydrolyze with release of acetic acid, or neutral systems which hydrolyze with release of oxime compounds, such as, e.g., butan-2-one oxime (or methyl-ethyl-ketoxime, MEKO, respectively).
For the desired broad application range of the silicone rubber materials, these should adhere to as many surfaces as possible, such as, e.g., to wood, varnished wood, glazed wood, metals, such as steel, aluminium, powder-coated aluminium, glass, plastics, such as polyvinylchloride (PVC), polyamide, concrete, etc. In addition, the silicone rubber materials should be stable in storage within an ordinary cartridge, i.e. their properties should not change in dependence of the storage time; Preferably, both in a cartridge sealed after filling, and in a cartridge that is already opened and/or partially emptied. Finally, the polymerization product obtained after complete cross-linking (cure) of the silicone rubber material should be transparent or clear, respectively.
Apart from the appropriate selection of the actual components of the polymer, such as cross-linker and polyorganosiloxane, for controlling the polymerization rate and/or polymerization degree, a catalyst is usually added, by which important product properties of the silicone rubber materials are influenced, such as, e.g., the skin formation time (i.e. the time, at which a first complete skin is formed on an applied material), the tack free time (i.e. the time, after which the material no longer exhibits tackiness), the complete cure (i.e. the time, at which the polymerization is completed), etc. For example, the following properties are expected from commercial silicone sealing compounds: a skin formation time of 5 to 15 minutes, a tack free time of 15 to 120 minutes, and a complete cure of maximal 7 days upon application with a height of 10 mm. Further information regarding the skin formation time, tack free time and complete cure can be taken, e.g., from the “Praxishandbuch Dichtstoffe” (3rd edition 1990), which has been published by the Industrieverband Dichtstoffe e.V. (IVD).
Up to now, a metalorganic catalyst was used as a catalyst for silicone rubber materials, such as one ordinarily used for polysiloxanes cross-linking by condensation, in particular a catalyst on basis of a tin-organic compound, such as, e.g., an alkyl-tin-carboxylate, especially dibutyl-tin-dilaurate and dioctyl-tin-dilaurate. However, such tin-organic compounds exhibit toxicological properties, which have led to restrictions of their use in commercially available products (cf. EU directive 76/769/EWG of 28.05.2009).
As a replacement, another catalyst, which does not comprise a tin-organic compound, was used for the cross-linking of polysiloxanes in the art so far.
For example, a titanium-based compound can be used as catalyst, as described, e.g., in EP 1 230 298 A1 and EP 2 290 007 A1. However, it is known that a catalyst on basis of a titanium compound can result in yellowing and/or surface tackiness in the products, and has a slow vulcanizing rate, a wanting storage stability, and an incompatibility with current adhesion promoters (coupling agents) on basis of aminosilanes.
Moreover, EP 1 230 298 A1 describes a catalyst on basis of amines and metal salts of tin, zinc, iron, lead, barium, and zirconium, as well as on basis of chelates of tin. This catalyst results in products having a little yellowing, but is a slow catalyst according to EP 2 290 007 A1.
Therefore, EP 2 290 007 A1 proposes a catalyst on basis of metal compounds of the I. and II. main and transition groups, i.e. Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Cu, Ag, Au, Zn, Cd, and Hg, in form of pure carboxylates, which result in a product having an acceptable hardening progress upon addition of an acid co-catalyst in form of an organic or inorganic acid.
EP 2 290 007 A1 describes the use of this catalyst in silicone rubber materials with cross-linkers (hardeners), which hydrolyze with release of acid (acetic acid) or neutral (alcohols or MEKO, respectively) compounds, respectively, in particular by using a catalyst made from a Li carboxylate or from a Sr carboxylate, respectively.
The use of a catalyst on basis of Li, Na, K, Mg, Ca, Sr compounds without an additional simultaneous use of an acid co-catalyst is described in EP 2 280 041 A1. Therein, it is further described that in particular the use of Octasoligem lithium or Octasoligem strontium in combination with alkoxy, acetoxy or oximo RTV-1 results in products having desired properties.